Quality center-based care has been shown to help prepare young children for future academic success, and this is especially true for children from diverse low-income families. One of the reasons why center-based programs like Head Start and state-funded prekindergarten may be effective and of high quality is because of the use of curricula, which guide interactions and activities in the classroom. However, the literature has not sufficiently provided evidence of features of specific curricular packages that attend to all facets of children's development and meet the needs of children with different backgrounds. This mixed-methods study addresses an important topic of interest to the Administration for Children and Families, the Child Care Development Fund, and the Office of Head Start: curriculum as a key feature of high-quality center-based care and education for young children. Specifically, the study seeks to understand the influence of various preschool curricula on children's school readiness skills, for whom they work best, and how they are implemented in the classroom by answering: (1) Are commonly used preschool curricula packages differentially related to children's school readiness skills? (2) What is the effect of academically targeted curricula on the distribution of children's academic achievement? (3) How do teachers implement the same curriculum across classrooms and how might they adapt the curriculum to address children's diverse needs? To address the research questions, quantitative analysis of two national datasets (Head Start Family and Child Experiences Survey 2009 Cohort and Preschool Curriculum Evaluation Research Initiative Study) containing rich data on child care, curricula use, and school readiness outcomes will be analyzed. I will investigate whether curricular packages are related to children's school readiness outcomes and whether the treatment effects of some packages vary along the distribution of children's outcomes. This will be followed by a qualitative study in which I conduct observations in local Orange County, California Head Start centers to better understand whether the heterogeneity of implementation by teachers using the same curriculum contributes to variation in its effects. The results of this study will further the field's understanding of curriculum as an important feature in center-based care, both nationally and locally, and has implications for promoting school readiness skills for linguistically and culturally diverse low-income and disadvantaged children. Further, understanding variation in curricula effects is critical for facilitating the most efficient use of limited resources, by informing decisions about how best to targeted specific curricular programs, and suggesting ways to improve the design or implementation of the programs for center-based child care settings.